This invention relates generally to pollution reduction systems, and more particularly to methods and apparatus for monitoring and controlling selective catalytic reduction emission control systems.
Two issues faced by owners and operators of facilities equipped with Selective Catalytic Reduction (SCR) systems include proper control of the ammonia distribution across the face of the catalyst (or to different SCR modules) and deterioration of the SCR catalyst.
An SCR is capable of achieving high levels of NOx destruction by injecting controlled quantities of ammonia into the NOx-laden gas stream and passing the mixture across a catalyst at a controlled temperature. The primary NOx destruction reaction in an SCR can be described as:4NO+4NH3+O2→4N2+6H2OAs indicated by this reaction, optimum NOx destruction occurs when the molar flow of ammonia is essentially equal to the molar flow of NO. If there is a deficiency in ammonia, then the NOx will not be completely destroyed. If there is excessive ammonia flow, then the ammonia (considered a hazardous air pollutant in several states) will pass through the system unreacted. This is referred to as ammonia slip. In general, SCR systems are equipped with hardware to continuously measure the molar flow rate of NOx coming to the catalyst. That data are used to calculate and control the appropriate amount of ammonia to inject into the system at any point in time. A grid of spray nozzles (referred to as an Ammonia Injection Grid or AIG) is provided to distribute the ammonia across the flow field and (hopefully) to provide the proper mixture of NH3 and NOx at the face of the catalyst. One of the first challenges faced during start up of a new SCR system is to achieve proper balancing of the AIG. The optimal approach for meeting that start up challenge is to gather data on the NOx and NH3 concentration from multiple locations at either the inlet or exit of the SCR catalyst.
Beyond the initial balancing, SCR systems can experience drift in the AIG performance. This can be caused by many system variables such as fouling of the injection nozzles, plugging of the ammonia transport lines, or shifts in the spatial distribution of the inlet NOx at the inlet to the SCR (or between different SCR modules). To identify that the AIG balance has drifted and to provide guidance for adjusting the AIG requires gathering of continuous data similar to that suggested above for initial AIG tuning.
Also, the reactivity of an SCR catalyst degrades over time. Typically, catalyst performance will remain acceptably high for periods of 3 to 10 years but occasionally the reactivity can decline precipitously. A sharp drop in reactivity can occur due to many factors including catalyst poisoning or delamination of a wash coat type catalyst. Data are required to track the long-term performance of the catalyst and to discriminate the root cause of falling NOx destruction efficiency between poor AIG performance, by passing of the catalyst, or loss of catalyst reactivity.
At least one known method for initial balancing of AIG systems is based on measurement of only NOx concentration distribution at an outlet of the catalyst, measurement of NOx destruction efficiency, or manual measurement of ammonia. This method has proven satisfactory for SCR systems that operate at sub-stoichiometric ammonia levels, but is less satisfactory for systems that operate at near 1:1 inlet NOx to ammonia ratio or for facilities with stringent ammonia slip regulatory limits. The known method also does not provide continuous monitoring to detect deterioration in AIG balancing.
Also, at least one known method physically extracts catalyst samples on a regular basis and subjects the catalyst to reactivity testing at a remote laboratory, but does not monitor catalyst performance during yearlong periods between major plant outages. Some indication of catalyst performance is provided through continuous measurement of overall NOx destruction efficiency but those data cannot distinguish the impacts of AIG tuning from catalyst reactivity.